1. Field of the Invention
This invention variously relates to source reagent compositions for forming non-linear optically active metal borate films on substrates by metalorganic chemical vapor deposition (MOCVD), to a method of forming a non-linear optically active metal borate film on a substrate by MOCVD utilizing such source reagent compositions, and to devices comprising non-linear optically active metal borate films.
2. Description of the Related Art
Nonlinear optical (NLO) materials have great potential in integrated optical devices for data storage, high-speed laser printers, large-area displays and communications. Many organic and inorganic materials have the requisite polarizability to be useful in NLO applications, but various parameters define the merit of the materials in a given device, including their ease of fabrication. Although organic nonlinear materials are currently being extensively investigated for inherent advantages in structural processability, difficulties with purification, crystallization and stability have not been sufficiently overcome for commercial application.
Inorganic materials have been more extensively studied, and examples which have found their way into commercial devices include lithium niobate (LiNbO.sub.3), potassium niobate (KNbO.sub.3), potassium titanyl phosphate (KTP), lithium triborate (LiB.sub.3 O.sub.5 or LiBO), ahd barium metaborate (BaB.sub.2 O.sub.4, or BBO). Although these materials are well known in bulk single crystal form, only recently have they been considered in thin films for integrated circuit applications.
A number of potential advantages are apparent when considering thin films for NLO applications. Particularly in second harmonic generation (SHG), the use of NLO thin film waveguides is very attractive. For example, the thin film is inherently a planar waveguide, thereby eliminating other processing methods for formation of waveguides which can be detrimental to SHG coefficients. Thin films can also be deposited with high integrity morphology, which improves the damage thresholds, and reduces optical losses. Nonlinear morphologies can actually be more efficient in waveguide structures than in bulk materials, since large power densities can be maintained over longer interaction lengths. The waveguide can also be engineered such that large differences in refractive index confines the wave to the NLO material and thereby enhances the SHG efficiency. Another potential advantage is the ease of fabricating periodically poled structures on a thin film for quasi-phase matching. Furthermore, the scale up and processing costs of thin film technology offer great advantage over bulk crystal fabrication.
A film deposition method that has demonstrated considerable advantages over other methods is MOCVD. MOCVD is the method of choice for manufacturing many kinds of semiconductor materials in industry, thus facilitating the integration of a borate deposition process. The primary attraction of this technique for NLO materials is the potential for in-situ growth of epitaxial films. Although more complicated than sol-gel processes, MOCVD is viewed as the technique that will ultimately be used for device manufacture if epitaxial, phase-pure thin films can be formed at temperatures compatible with other device materials. Advantages of MOCVD for thin film oxide growth include the use of inexpensive apparatus, high throughput, and adaptability to large area conformal coverage.
Concerning specific applications of NLO materials, the use of NLO crystals to alter the frequency of incident laser radiation is currently the preferred method for obtaining laser radiation in the ultraviolet. Laser emission in this region is important for communications and high density data storage applications due to the shorter wavelength and smaller bit size that can be achieved. Using NLO materials, commercial visible wavelength lasers such as Nd:YAG can produce wavelengths in the UV region with high power density. The NLO materials considered most suitable for UV generation include .beta.-BaB.sub.2 O.sub.4 (BBO), LiB.sub.3 O.sub.5, urea, and L-arginine phosphate (J. T. Lin, Opt. Quant. Elect. 22, S305, (1990)). The single-crystal values of the relevant .chi..sup.eff.sup.(2) index for these materials are .beta.-BaB.sub.2 O.sub.4, 4.6 pm/V; LiB.sub.3 O.sub.5, 2.52 pm/V; urea, 2.61 pm/V; and L-arginine phosphate, 1.65 pm/V. Due to the acceptable .chi..sub.eff.sup.(2) value, transparency in the ultraviolet, and high damage threshold, BBO is being actively investigated for applications in the ultraviolet region. Thus, BBO single crystals are used in commercial systems as frequency doublers and triplers for the 1.06 .mu.m output of Nd:YAG lasers (CSK Optronics, 5519 Grosvenor Blvd., Los Angeles, Calif., 90066, SuperTripler 8310 series fact sheet). Crystals such as LiIO.sub.3, LiNbO.sub.3, KNbO.sub.3, and KTP have higher .chi..sub.eff.sup.(2) values than BBO but are not sufficiently transparent in the UV region (0.1-0.01 .mu.m). For example, LiIO.sub.3 is transparent to 300 nm whereas BBO is transparent to 190 nm. Other factors which make BBO attractive are the high damage threshold of &gt;3 GW/cm.sup.2 compared to 1 GW/cm.sup.2 for most of the aforementioned materials.
Thin films of NLO materials have been heavily studied for applications as frequency converters, waveguides, switches, and storage devices. Waveguides of LiNbO.sub.3 are commercially available (P. F. Bordui, and M. M. Fejer, in Annual Reviews of Materials Science, edited by R. A. Laudise, E. Snitzer, R. A. Huggins, J. A. Giordemaine, and J. B. Wachtman (Annual Reviews Inc., Palo Alto, 1993), Vol. 23, p. 321). High-quality thin films of BBO would be extremely attractive for integrated optical applications requiring UV waveguides. To date, oriented BBO thin films have been grown by pulsed laser deposition (R.-F. Xiao, L. C. Ng, P. Yu, and G. K. L. Wong, Appl. Phys. Lett. 67, 305 (1995)), and magnetron sputtering (H. B. Liao, R.F. Xiao, P. Lu, G. K. L. Wong, and J. Q. Zheng, J. Vac. Sci. Technol. A 14, 2651, (1996)) and less well-oriented films by sol-gel processing (S. Hirano, T. Yogo, K. Kikuta, and K. Yamagiwa, J. Am. Ceram. Soc. 75, 2590 (1992)).
Critical to optical properties of NLO thin films are homogeneity and morphological integrity. Potential contributions to high loss in films tested for waveguide applications appear to be scattering centers caused by particles or inhomogeneities, or by substrate attack during deposition and annealing. The latter lead to scattering losses from a rough interface. Clearly it would be advantageous to improve the process by using MOCVD, since deposition in situ, at lower temperatures, has better potential for preservation of the interface region.
It therefore is one object of the present invention to provide an MOCVD process producing high quality NLO materials.
It is another object of the present invention to provide precursor compositions for MOCVD formation of NLO materials.
Other objects and advantages will be more fully apparent from the ensuring disclosure and appended claims.